Mary E. MacKay

Origin of bottom-simulating reflectors: Geophysical evidence from the Cascadia accretionary prism

Vertical seismic profile (VSP) data from two drill sites on the Cascadia margin show low-velocity zones, indicative of free gas, be. neath a bottom-simulating reflector (BSR). Offshore Oregon, at Ocean Drilling Program (ODP) Site 892, velocities drop from an average of 1750 m/s above the BSR to less than 1250 m/s below it. Sonic logs confirm that seismic velocity in the sediments adjacent to the borehole is less than that of water for at least 50 m beneath the depth of the BSR at this site. Similarly, at ODP Site 889 offshore Vancouver, velocities range from 1700 to 1900 m/s in the 100 m above the BSR and drop abruptly to 1520 m/s in the 15 m just beneath it. The low velocities observed beneath the BSR are strong evidence for the presence of 1%-5% free gas (by volume). The BSR at these two sites results from the contact between gas-free sediments containing a small quantity of hydrate above the BSR and a low-velocity free-gas zone beneath it. Although the BSR is associated with the base of the hydrate stability field, hydrate appears to account for relatively little of the velocity contrast that produces the BSR. Velocity above the BSR at Site 889 is only about 100 m/s greater than that expected for sediments of similar porosity. Sediments above the BSR at Site 892 appear to have normal velocity for their porosity and may, contain little hydrate.

Landward vergence and oblique structural trends in the Oregon margin accretionary prism: Implications and effect on fluid flow

Abstract The central Oregon margin spans a regional transition in accretionary structures from seaward-verging in the south to landward-verging in the north. New multichannel seismic (MCS) data image both landward- and seaward-vergent provinces along the central and northern Oregon margin. Landward-vergence is characterized by a deep decollement, with deformation distributed across a broad lower continental slope in a coherent structural style. In the landward-vergent area, virtually all 4 km of incoming trench sediments overthrust the preceding thrust sheet, forming fault-bend folds and a distinctive ridge/trough morphology. This style of landward-vergence is not explained by existing models. In contrast, seaward-vergence correlates with a shallower decollement, approximately 1.4 km above the oceanic crust, and a more intense style of deformation within a narrower slope. Initial thickening of the trench sediments occurs across a well-developed protothrust zone. The frontal thrust forms a ramp-anticline that is cut by a prominent backthrust. Previously observed seafloor vent sites in both regions correlate with thrusts that exhibit high-amplitude, reversed-polarity reflections suggestive of enhanced porosity along the faults. Potential fluid sources and migration paths are strongly influenced by changes in the level of the decollement and vergence along the margin. Abrupt changes in structural style occur both along strike and updip, and are bounded by two sets of oblique-slip faults. Three NW-striking left-lateral faults are imaged in both MCS and SeaBeam data. Plunging anticlines developed along the NW-striking faults are venting fluids and were previously interpreted as mud volcanoes. The deformation front is locally disrupted where these faults intersect the prism, but they appear to have limited influence on the structural evolution of the prism. In contrast, the NE-striking right-lateral faults are confined to the deforming sediments of the upper plate. These faults interact with the thrusts within the prism, forming a rhomboidal pattern of three-dimensional deformation.

Structural variation and landward vergence at the toe of the Oregon accretionary prism

The Oregon margin near 45°N spans a regional transition in structural style from seaward vergence in the south to landward vergence in the north. This variation probably reflects a regional change in both sediment type and rate of deposition that affects the potential for overpressure in the sediments. Structural style within the survey area shows a gradual northward transition from seaward to landward vergence and to lower slopes within the landward vergent area, suggesting a northward decrease in basal shear stress. Superimposed on this gradational variation are abrupt changes in structural style that correlate with NW striking strike-slip faults in the Cascadia Basin. Because sediments thicken toward the east, translation along the strike-slip faults results in juxtaposition of sediments with different physical properties and loading histories. In addition, the faults themselves may act as fluid conduits, resulting in stepwise changes in pore pressure on the decollement and concomitant change in structural style across the faults. Although the Oregon-Washington margin is dominated by landward vergence, landward vergence has not been adequately explained by theoretical models or replicated in experimental models because of a simple omission in the boundary conditions of the underlying conceptual model. Landward vergence requires not only low basal shear stress but also an arcward dipping decollement (and to a lesser degree, a relatively strong wedge). In order for landward vergence to predominate, these three factors must combine in such a way that the backward verging thrust planes are favored.

Amplitude versus offset modeling of the bottom simulating reflection associated with submarine gas hydrates

Abstract A bottom simulating seismic reflection (BSR) that parallels the sea floor occurs worldwide on seismic profiles from outer continental margins. The BSR coincides with the base of the gas hydrate stability field and is commonly used as indicator of natural submarine gas hydrates. Despite the widespread assumption that the BSR marks the base of gas hydrate-bearing sediments, the occurrence and importance of low-velocity free gas in the sediments beneath the BSR has long been a subject of debate. This paper investigates the relative abundance of hydrate and free gas associated with the BSR by modeling the reflection coefficient or amplitude variation with offset (AVO) of the BSR at two separate sites, offshore Oregon and the Beaufort Sea. The models are based on multichannel seismic profiles, seismic velocity data from both sites and downhole log data from Oregon ODP Site 892. AVO studies of the BSR can determine whether free gas exists beneath the BSR if the saturation of gas hydrate above the BSR is less than approximately 30% of the pore volume. Gas hydrate saturation above the BSR can be roughly estimated from AVO studies, but the saturation of free gas beneath the BSR cannot be constrained from the seismic data alone. The AVO analyses at the two study locations indicate that the high amplitude BSR results primarily from free gas beneath the BSR. Hydrate concentrations above the BSR are calculated to be less than 10% of the pore volume for both locations studied.

The effect of varying acquisition parameters on the interpretation of SIR-C radar data : The Virunga volcanic chain

Abstract In 1994, the Spaceborne Imaging Radar experiment C (SIR-C) acquired five data takes with different acquisition parameters over the Virunga volcanic chain, Zaire. Changes in incidence angle (20.0–48.1°), look direction (west- and east-looking), and pulse bandwidth (10 MHz, 20 MHz, and 40 MHz) affect the geologic interpretations drawn from these data. We examine differences in spatial distortion of features near the summit of Nyiragongo volcano, finding that opposite look direction data are helpful in interpreting volcanic features, such as the shape of small craters, in areas of high relief. The relative brightness of lava flows and vegetation change (and actually reverse) for different wavelengths and polarizations. Although L-band data are best for mapping the outline of the flows, C- and X-band cross-polarization data show differences in backscatter intensity between and within flows. In addition, our ability to discriminate lava flows and vents from surrounding vegetation is strongly influenced by pulse bandwidth (resolution) and angle of incidence. Pulse bandwidth is the most important parameter in mapping small topographic features such as volcanic cones and a set of particularly thick flows on the flanks of Karisimbi volcano. These examples illustrate important considerations of the tradeoffs and priorities in acquisition parameters for future missions such as ENVISAT, as well as potential pitfalls for interpretation of existing data.

Large-scale duplexes within the New Britain Accretionary Wedge: A possible example of accreted ophiolitic slivers

A multichannel seismic line acquired in the western Solomon Sea images reflectors within the New Britain accretionary wedge which we interpret as accreted duplexes of the downgoing oceanic plate. The seaward edge of the largest duplex is located just 6 km from the toe of the accretionary wedge suggesting recent incorporation into the wedge. These data therefore provide an excellent opportunity to determine the dynamics of oceanic basement accretion. The oceanic basement is represented by a high-amplitude, low-frequency reflector that can be traced up to 40 km arcward of the toe of the accretionary wedge on several seismic lines in this survey. Another prominent reflector, similar in character to the basement reflector, lies approximately one second higher in at least one seismic section. This reflector is likely to be the top of a basement duplex. Structures imaged above the basement duplex define a preduplex accretionary wedge, whereas those at the toe indicate a new episode of postduplex accretion. We have tested this interpretation using both magnetic modeling and interval velocity analysis. Magnetic models of the wedge that include basement duplexes provide a good match to the observed magnetic data and provide limits on the size and interval velocities of the slivers. Interval velocities between the two strong reflectors, calculated using stacking velocities and constrained through magnetic modeling, exceed 5000 m/s and are consistent with our interpretation of ophiolitic slivers within the accretionary wedge.

A left‐lateral strike‐slip fault seaward of the Oregon Convergent Margin

We have mapped a recently active left-lateral strike-slip fault (the Wecoma fault) on the floor of Cascadia Basin west of the Oregon convergent margin, using SeaMARC I sidescan sonar, Seabeam bathymetry and multichannel seismic and magnetic data. The fault intersects the base of the continental slope at 45°10′N and extends northwest (293°) for at least 18.5 km. The faults western terminus was not identified, and the eastern end of the fault splays apart and disrupts the lower continental slope. The fault extends to the base of the 3.5-km-thick sedimentary section and overlies a basement discontinuity that may be related to movement along the Wecoma fault. Prominent seafloor features crosscut by the fault individually display between 120 and 2500 m of left-lateral separation, allowing the general history of fault motion to be evaluated. The faults average slip rate since 10–24 ka is inferred to be 5–12 mm/yr, based on the age of an offset submarine channel. Surficial structural relationships, in conjunction with the maximum inferred slip rate, indicate that fault movement initiated at least 210 ka and that the fault has been active during the Holocene.

The case against porosity change: Seismic velocity decrease at the toe of the Oregon accretionary prism

As sediments are incorporated into the first thrust sheet of the landward-vergent Oregon accretionary prism they undergo a decrease in interval velocity. This trend is opposite that observed at other accretionary prisms, where an increase in velocity normally accompanies the loss of porosity during deformation. Velocities were obtained from focusing analysis and iterative prestack depth migration of multichannel seismic data for four stratigraphically defined intervals. The shallowest interval (